1. Field of the Invention
The present invention relates to methods of measuring and controlling an activity of a zinc phosphate treatment solution for forming a zinc phosphate coating on a surface of an aluminum member or an aluminum alloy member, and more specifically, it relates to methods of measuring and controlling an activity of a zinc phosphate treatment solution which contains complex hydrofluoric acid as an active material.
2. Description of the Background Art
In the field of a material for an automobile body or the like, employment of aluminum or an aluminum alloy material is carried out in practice or studied for the purpose of reduction in weight. In such use, a coating film may be formed by electrodeposition coating such as cationic electrodeposition coating. In order to improve the coating film in adhesion and corrosion resistance, a zinc phosphate coating is generally formed on a surface of the material as a pretreatment for the electrodeposition coating. Such a zinc phosphate coating is formed on the aluminum material by a method such as that disclosed in Japanese Patent Publication No. 45-16566 (1970), for example. According to this method, a zinc phosphate coating is formed by a zinc phosphate treatment solution containing zinc ions, nitrate ions, phosphate ions, SiF.sub.6 ions, iron (II) ions and F ions as main components. When such a zinc phosphate treatment solution contains SiF.sub.6 ions and F ions in the form of H.sub.2 SiF.sub.6 and HF, the surface of the aluminum material is etched and activated through the following chemical formulas: EQU 2Al+3H.sub.2 SiF.sub.6 .fwdarw.Al.sub.2 (SiF.sub.6).sub.3 +3H.sub.2 EQU Al+3HF.fwdarw.AlF.sub.3 +3/2H.sub.2
As understood from the above chemical formulas, HF and H.sub.2 SiF.sub.6 react with aluminum in the zinc phosphate coating treatment, to form respective aluminum salts. Consequently, free HF and H.sub.2 SiF.sub.6 concentration levels are reduced by the treatment, leading to reduction of activity levels with respect to the aluminum material. In order to control these activity levels so that the zinc phosphate treatment solution regularly has constant activity, it is necessary to measure the free HF and H.sub.2 SiF.sub.6 concentration levels.
The HF concentration can be measured with a silicon electrode meter, for example. U.S. Pat. No. 3,S29,587 discloses a method of measuring free HF concentration with such a silicon electrode meter. According to this method, an electrolytic current with a constant voltage is fed across an inactive cathode and an anode of P-type silicon provided in an electrolytic cell which is filled up with a target solution, for measuring the HF concentration in the target solution from the current density. As to F.sup.- ions contained in the zinc phosphate treatment solution, it is possible to measure the concentration by diluting the treatment solution containing fluorine ions to about 20 to 200 times in volume and employing a fluorine ion electrode, as disclosed in Japanese Patent Laying-Open No. 60-251280 (1985).
However, the concentration of complex hydrofluoric acid such as H.sub.2 SiF.sub.6 cannot be measured by the aforementioned silicon electrode meter or the fluorine ion electrode, and there has been no simple method of measuring the same.
When H.sub.2 SiF.sub.6 reacts with aluminum, the corresponding salt is produced, as hereinabove described. Therefore, the treatment solution which is reduced in activity by the treatment contains H.sub.2 SiF.sub.6 and Al.sub.2 (SiF.sub.6).sub.3. Activity of such a treatment solution must be measured in the following manner, for example:
First, the zinc phosphate treatment solution of pH 2.8 to 3.5, for example, is diluted with a total ionic strength adjustment buffer solution (TISAB), and the total fluorine concentration in terms of F is measured by a fluorine ion electrode method.
The TISAB as employed is prepared by dissolving 58 g of sodium chloride for adjusting total ionic strength, 1 g of sodium citrate for masking polyvalent cations and 50 ml of glacial acetic acid for pH adjustment in about 500 ml of pure water, thereafter adding a 5N sodium hydroxide solution to the mixture with adjustment of its hydrogen ion concentration to pH 5.3 through a pH meter, and further adding pure water to the mixture to attain a quantity of 1000 ml.
It is possible to obtain the free H.sub.2 SiF.sub.6 concentration by obtaining total fluorine concentration in the aforementioned manner and further measuring Al concentration in the zinc phosphate treatment solution by atomic absorption analysis or the like.
In order to return the reduced activity to the original level after such measurement, the complex hydrofluoric acid is supplemented in general. When the complex hydrofluoric acid is newly supplemented in such a manner, it is necessary to measure the Si concentration, in addition to the aforementioned total fluorine concentration and Al concentration. Consequently, the conventional method of measuring activity is extremely complicated and requires a long time.
Thus, generally awaited is a method which can simply and quickly measure activity of a zinc phosphate treatment solution which contains complex hydrofluoric acidas an active material, together with a method of controlling activity of the zinc phosphate treatment solution through this method.